This invention relates to a polymer composition for molded articles exhibiting desirable combinations of stiffness and impact resistance. More particularly, this invention relates to a composition comprising an engineering polymer and a mineral filler.
So-called engineering polymers, such as polyamides, polyacetals, and polyesters, are known in the art to provide useful combinations of stiffness and toughness at room temperature and under moderate deformation rates, and are in very widespread commercial use. However, they become brittle under more extreme conditions such as the very high deformation rates associated with high impact rates such as that associated with the well known Notched Izod Impact Resistance test, ASTM D256 which is in widespread commercial use for evaluating the suitability of various polymeric materials for various practical applications. Numerous strategies have been employed to provide higher impact toughness but these have met limited success because they result in trade-offs which are often undesirable.
For example, it is known that plasticizing a semi-crystalline engineering polymer provides limited improvement in impact toughness but with a highly undesirable loss in modulus and strength. On the other hand, incorporation of glass fibers improves strength, modulus, and impact toughness, but generally results in anisotropic molded parts and undesirable levels of mold shrinkage. So-called rubber toughening is well known to improve toughness with relatively less effect on other properties but rubber toughening is expensive, increasing the cost of the resin. It is thus desirable to provide a means for improving the impact toughness of engineering polymers while minimizing the need to make those various tradeoffs.
Polyamide nylons, such as nylon 66 or nylon 6, are very well known in the art and have been in wide spread commercial use for 60 years. Nylons are condensation polymers of amines and acids or anhydrides having in common that the resulting polymer backbone contains repeating units with a nitrogen atom in the backbone.
Polyacetals are well known in the art, and are in widespread commercial use, principally in the form of polyoxymethylene homopolymer and copolymers, Polyoxymethylene homopolymer is generally formed by polymerizing formaldehyde or trioxane, the cyclic ether form of formaldehyde. Copolymers are generally formed by combining formaldehyde with various cyclic ethers such as ethylene oxide or 1,3-dioxolane to form a polymer chain having two or more methylene groups directly adjacent to one another, thereby improving the thermal stability over that of the hompolymer.
Polyesters are condensation products of a diacid and a glycol. While numerous polyesters are known in the art, polyethylene terephthalate is best known. Polyesters, like polyacetals, have polymer backbones with repeat units having oxygen atoms in the polymer backbone.
The methods of preparation for nylons, polyacetals, polyesters, and other well-known engineering polymers are described by Brydson in Plastics Materials, 5th edition, Butterworth-Heinemann, Oxford (1991), and in the references therein provided.
Representative of the modifications to engineering polymers which are commercially available and the properties obtained are those resin grades listed in Table 1. Shown are the notched Izod impact resistance and flexural modulus of several grades of Delrin(copyright) polyoxymethylene homopolymer, Zytel(copyright) nylon 66, and Minlon(copyright) mineral-filled nylon 66, all available from the DuPont Company, Wilmington, Del. Flexural modulus was determined according to ASTM D790, and notched Izod impact resistance was determined according to ASTM D256.
Referring to the data in Table 1, a Zytel(copyright) 408L plasticized nylon resin exhibits a 330% improvement in impact strength but at about 230% reduction in stiffness in comparison to the general purpose Zytel(copyright) 101 resin. Plasticizers in general improve processibility, but degrade numerous other mechanical properties. Elastomer filled Zytel(copyright) ST801 known as a xe2x80x9crubber toughenedxe2x80x9d or xe2x80x9csuper-toughxe2x80x9d nylon composition exhibits about a 1600% improvement in toughness, but at about a 40% loss in stiffness in comparison to the general purpose. Zytel(copyright) 101 resin. Additionally, rubber toughened nylons are expensive to produce.
With continuing reference to Table 1, blending short glass fibers in a resin composition (e.g., Zytel(copyright) 71G33L and Delrin(copyright) 525GR) provides about 135% improvement in toughness and about a considerable increase in stiffness over the unmodified resins. However, glass fibers significantly reduce the moldability of the resulting resin and may lead to property anisotropy, uneven shrinkage, and part warpage. Mineral fillers provide similar improvements in stiffness to the resin but usually with a reduction in toughness, even though processibility and product isotropy are improved.
U.S. Pat. No. 4,399,246, to Hyde discloses polyamide compositions comprising 50 to 75 parts of resin, 25 to 50 parts of mineral filler, 0.2 to 0.9 parts of aminofunctional silane, and 0.2 to 0.9 parts of a sulfonamide. The mineral fillers include calcined clay, wollastonite, and talc in the size range of 0.2 to 2 micrometers. The stiffness of filled nylon 66 was 5865 MPa while Izod impact resistance was about 65 J/m.
U.S. Pat. No. 4,740,538, to Sekutowski discloses a nylon composition containing a kaolin filler precoated with an amino functional silane, the composition further containing a phenol or triethanolamine as an impact modifier.
Wu et al, Proc. Inter. Conf. Pet. Ref. and Petrochem. Proc., 2, pp 802ff (1991) discloses employing a rubber coated CaCO3 to effect improvements in impact resistance of nylon 6.
U.S. Pat. No. 5,571,851 to Freeman et al. discloses an Izod impact of 42.6 J/m (0.76 ft-lbs/in) and a flexural modulus of 5620 MPa (816 ksi) when a combination of 25% stearylsilane and 75% aminosilane is incorporated into a composition of nylon 66 and calcined clay comprising 40% of calcined clay. When only the stearylsilane is employed both flexural modulus and Izod impact resistance are significantly lower.
It is known in the art to incorporate fatty acids, particularly stearic acid, into mineral filled polyolefin compositions. For example, U.S. Pat. No. 4,795,768 to Ancker et al. discloses a composition consisting of high density polyethylene filled with 50 wt-% of a 3.5 xcexcm CaCO3 pre-treated with 2% by weight of isostearic acid. Izod impact was decreased by about 8% with respect to the unfilled polymer while flexural modulus was increased by about 150%.
Orange, 10th Int. Conf. Deformation, Yield, and Fracture of Polymers, Inst. of Mat., pp. 502ff, (1997) discloses filled polypropylene compositions containing 10% by volume of a 0.1 xcexcm and 2 xcexcm CaCO3 both stearic acid treated and untreated. The compositions containing the stearic acid treated fillers exhibited fracture toughness higher than the unfilled polymer and somewhat higher stiffness. The composition containing the 2 xcexcm untreated filler was similar to the treated composition, but that containing the 0.1 xcexcm untreated filler exhibited a 50% reduction in fracture toughness and about a 45% increase in stiffness.
Suetsugu, The Polymer Processing Society, (1990), discloses an increase of notched Izod impact resistance of 230% in a high molecular weight polypropylene composition containing 30% by weight of stearic acid treated 4.3 xcexcm CaCO3.
U.S. Pat. No. 3,926,873 to Aishima et al. discloses compositions comprising inorganic fillers, unsaturated carboxylic acids, and nylon 6 and nylon 66 polymers. Improvements in Izod impact resistance of less than 50% are realized while flexural modulus is increased by 50%. The process of Aishima requires a preliminary reaction step between the filler and the unsaturated carboxylic acid, followed by melt processing with the polymer in the presence of a free-radical generator.
The differences between saturated and unsaturated fatty acids in their interaction with mineral particles is disclosed in Ottewill et al., J. Oil Colour Chemists Assn, 50:844(1967).
Flexman in Toughened Plastics I, C. Keith Riew and Anthony J. Kinloch, editors, American Chemical Society, Washington, 1993, shows that the fracture mechanics of polyacetals differs considerably from that of polyamides.
In one aspect of the present invention, there is provided a composition comprising an engineering polymer having a backbone comprising repeat units, at least 80 mol-% of which repeat units comprise one or more oxygen or nitrogen atoms disposed in said backbone; about 1%-30% by volume of a mineral filler having an aspect ratio of 5 or less, the filler having an average equivalent spherical diameter in the range of about 0.1 to less than about 3.5 micrometers, and a saturated organic acid, salt thereof, or a mixture thereof, at a concentration of at least about 0.5% by weight of the mineral filler.
In another aspect, the invention relates to a process for forming a composition, comprising the steps of combining a hydrocarbon polymer having a backbone comprising repeat units, at least 80 mol-% of which repeat units comprise one or more oxygen or nitrogen atoms disposed in said backbone, with a mineral filler having an aspect ratio (the average ratio of the largest to the smallest dimension of the filler particle) of less than 5, the mineral filler having an average equivalent spherical diameter in the range of about 0.1 to about 3.5 micrometers, and a saturated organic acid, salt thereof, or a mixture thereof, at a concentration of at least about 0.5% by weight of the mineral filler, the mineral filler and the hydrocarbon polymer being combined at a weight ratio given by the formula:
Wf/Wp=[VF/(1xe2x88x92VF)]xc2x7Df/Dp 
where Wf is the weight of the filler, Wp is the weight of the polymer, VF is the desired volume fraction of filler, in the range of about 0.01-0.3, Df is the density of the filler, and Dp is the density of the polymer;
heating the combination to a temperature above the melting point of the hydrocarbon polymer to form a molten composition;
mixing the molten composition to provide a homogenous melt; and, cooling the molten composition.